The artists tax excemption, which has been mentioned by the commision on taxation, only covers royalty payments on original works, so for musicians, it only covers payment associated with material which they have written, not fees for playing, so doesn't apply to all musicians, or even for those musicians who get it, it wouldn't be on all their income.
Therefore a successful wedding / covers band would not get an tax excemption
at all.
Of course, cavaet is right, there is no average anything for such things